Scientists say a 2-billion-year-old rock from Mars that landed on Earth is quite different from other Martian meteorites.
They say it is older than most and contains more water. It's also similar to volcanic rocks examined by the NASA rovers Spirit and Opportunity on the Martian surface.
About 65 Martian meteorites have been recovered on Earth, mostly in Antarctica or the Sahara desert. The oldest dates back 4.5 billion years. The rest are 1.3 billion years old or younger.
The recently tested 2-billion-year-old rock is about the size of a baseball. It was donated to the University of New Mexico by an American who bought it from a meteorite dealer last year.
Researchers performed tests to confirm its age and origin. The findings were published online Thursday in the journal Science.